We do not have magical powers, of course. But, we can still change a few things around us, perhaps many things. Can you list a few things you can change around you, with no magic involved?

Many changes are taking place around us on their own. In the fields, the crops change from time to time. Sometimes, leaves fall from trees, change colour and dry out. The flowers bloom and then wither away. Are any changes happening in your body? Your nails grow, your hair grows, you grow taller and your weight increases as you grow. Did you realise earlier that so many changes are taking place around you all the time?

Can some of the changes be grouped together?

How can we group various changes? It might help, if we find some similarities between them.

6.1 Can All Changes Always Be Reversed?

Activity 1

Take a balloon and blow it. Take care that it does not burst. The shape and size of the balloon have changed (Fig. 6.1). Now, let the air escape the balloon.

Activity 2

Take a piece of paper and fold it as shown in Fig.6.2. You have changed the sheet of paper into a toy aeroplane. You may have lots of fun in flying this plane. Once you are tired of it, unfold the paper again.

Fig 6.2 A toy aeroplane made by folding paper

Activity 3

Take some dough and make a ball. Try to roll out a roti (Fig. 6.3). May be you are not happy with its shape and wish to change it back into a ball of dough again.

Fig 6.3 A ball of dough and a rolled out roti

Now, think about the three changes you observed in Activity 1, 2 and 3. What do they have in common?

Was it possible to get the balloon back to its original shape and size?

Was the size of the paper same as before and after making an aeroplane?

Was it possible to get back the ball of dough again?

What do you conclude? In each of the three activities, is it possible to get back to the material with which we started our activity? If the answer is yes, it means that the changes occurring in these activities can be reversed. Now, let us repeat the same activities with a difference.

Activity 4

Take the same balloon, which you used in Activity 1. Blow it to its full size and tie its mouth with a string tightly. Prick it with the pointed tip of your pencil. Oops! It burst.

Activity 5

Take the same piece of paper, which you used in Activity 2. Draw an aeroplane on it and cut along its outline (Fig.6.4).

Fig. 6.4 An aeroplane cut out of paper

Activity 6

Roll out a roti from the ball of dough again and bake it on a tawa (Fig.6.5).

Fig 6.5 A roti

Suppose, you are asked the same three questions which you answered after Activity 3. What would your answers be, now?

We see that, the changes which have occurred in the Activity 4, 5 and 6 can not be reversed.

You use a pencil and an eraser. With repeated use, their shape and size change. Can we reverse this change?
You must have seen a potter working on his wheel. He shapes a lump of clay into a pot. Can this change be reversed? He then bakes the pot in an oven. Now, can this change be reversed?

Some common changes are given in Table 6.1. Which of these changes, do you think can be reversed?

Table 6.1 Some common changes

We find that one way we can group changes is to see if they can be reversed.

6.2 Could There be Other Ways to Bring a Change?

We all have seen the tools which are used to dig the soil (Fig. 6.6 ). Have you ever seen how the iron blade in these tools is fixed to the wooden handle?

The iron blade of these tools has a ring in which the wooden handle is fixed. Normally, the ring is slightly smaller in size than the wooden handle. To fix the handle, the ring is heated and it becomes slightly larger in size (expands). Now, the handle easily fits into the ring. When the ring cools down it contracts and fits tightly on to
the handle.

Such a change is also used for fixing the metal rim on a wooden wheel of a cart as shown in Fig.6.7. Again the metal rim is made slightly smaller than the wooden wheel. On heating, the rim expands and fits onto the wheel. Cold water is then poured over the rim, which contracts and fits tightly onto the wheel.

Fig. 6.7 Cart wheel with metal rim fixed to it

When we heat water in a pan, it begins to boil after some time. If we continue to heat further, the quantity of water in the pan begins to decrease.

The water changes into its vapour. In Activity 7, Chapter 5 you have observed that water vapour gets changed into liquid water when it is cooled. We all have noticed melting of ice. Ice melts when it is heated. What does it change into? Is it possible to change this water back into ice?

Let us observe some more changes.

Activity 7

Take a small candle and measure its length with a scale. Now, fix it at a suitable place and light it. Let it burn for some time. Now blow out the candle and measure its length again (Fig.6.8).

Fig.6.8 Burning of a candle 

Can the change in the length of the candle be reversed? If we were to take some wax in a pan and heat it, can this change be reversed (Fig. 6.9)? 

Fig.6.9 Heating wax

Repeat Activity 7 with an incense stick. Wait till it burns away completely. What are the changes that occur in the incense stick? The stick burns to produce some new material. These are ash and some gases. We cannot see these gases but can sense them due to their pleasant smell. Can this change be reversed? And what about the change, which occurred in the matchstick you used for lighting the candle or incense stick?